1. Field of the Invention
This invention relates to an image-forming apparatus such as a copying machine and a printer in which an electrophotographic system is used and a developing device for developing an electrostatic latent image formed on an image supporting member, and more particularly, concerns a developing device in which a developer composed of two components of a toner and a carrier and an image-forming apparatus using such a device.
2. Description of the Related Art
Conventionally, with respect to a developing system for an electrostatic latent image formed on an image supporting member in the image-forming apparatus using the electrophotographic system, a one-component developing system that uses only the toner as a developer and a two-component developing system that uses a toner and a carrier have been known. In the one-component developing system, in general, the toner is allowed to pass through a regulating section that is constituted by a toner-supporting member and a regulating plate pressed onto the toner-supporting member so that the toner is charged and a desired thin toner layer is obtained; therefore, this system is advantageous from the viewpoints of simplifying and miniaturizing the device and of achieving low costs. In contrast, due to a strong stress in the regulating section, the toner is easily deteriorated to cause degradation in the toner charge-receiving property. Moreover, the toner regulating member and the surface of the toner-supporting member are contaminated by the toner and externally additive agents, with the result that the charge-applying property to the toner is lowered to cause problems such as fogging and the subsequent short service life of the developing device.
In comparison with the one-component developing system, the two-component developing system, which charges the toner through a friction-charging process upon mixing with the carrier, can reduce the stress, and is advantageous in preventing toner deterioration. Moreover, the carrier serving as a charge-applying material to the toner has a greater surface area so that it is relatively resistant to contamination due to the toner and externally additive agents, and is advantageous in prolonging the device service life.
However, even in the case of the two-component developer, the contamination on the carrier surface due to the toner and externally additive agents also occurs to cause reduction in the quantity of charge in toner after a long-term use, resulting in problems such as fogging and toner scattering; therefore, the device service life is not sufficient, and there is a strong demand for a longer service life.
With respect to a method for prolonging the life of the two component developer, Patent Document 1 has disclosed a developing device in which the carrier, alone or together with the toner, is supplied little by little, while a deteriorated developer having a reduced electrostatic charge property (simply referred to as “charge property”) is discharged in response to the supply so that the carrier is exchanged to prevent increase in the ratio of the deteriorated carrier. In this device, since the carrier is exchanged, the reduction in the quantity of charge in toner due to the deteriorated carrier can be suppressed in a certain level, making it possible to provide a long service life. However, since a mechanism for collecting the discharged carrier is required, and since the carrier is used as a consumable supply, problems arise in costs, environmental preservation, and the like. Moreover, since a predetermined number of printing processes need to be repeated until the ratio of the new and old carriers has been stabilized, there is a failure to maintain and effectively use the initial properties.
Patent Document 2 has disclosed a two component developer composed of a carrier and a toner to which particles that exert a charge property with a reverse polarity to the toner charge polarity are externally added, and a developing method using such a developer. In the developing method of Patent Document 2, the reverse polarity-chargeable particles are added in an attempt to add functions as a polishing agent and spacer particles, and it describes that by the effect of removing spent matters on the carrier surface, the degradation preventive effect is obtained. Moreover, it also describes that in the cleaning unit in the image supporting member, the cleaning property is improved, and that the polishing effect of the image supporting member is obtained. However, in the disclosed developing method, the amounts of consumption in the toner and the reverse polarity-chargeable particles are different depending on the image area rate, and in particular, in the case of a small image area rate, the consumption of the reverse polarity-chargeable particles becomes excessive, causing degradation in the carrier deterioration preventive effect in the developing device.
[Patent Document 1] Japanese Patent Application Laid-Open No. 59-100471
[Patent Document 2] Japanese Patent Application Laid-Open No. 2003-215855